But how seriously should the world take the north korean leader's threats? Abc's reena ninan reporting from the white house tonight. Reporter: Today's declaration by north korea that it's at war with the south -- Reporter: -- Had the white house and pentagon trying to bring tensions down a notch. I think you'd know if we were in a state of war with north korea. We believe that this is part of their overheated rhetoric, and it's something that we think they should avoid. Reporter: The talk of war comes two days after the u.S. Flew in b-2 stealth bombers as part of a joint training exercise with south korea. The message was clear. We have a range of options that we've worked with the south koreans to counter their provocations and threats. Obviously we hope never to have to put those plans into effect. Reporter: Little is known about the 30-year-old north korean leader, kim jong-un, who rose to power a year ago following his father's death. He now finds himself in a power struggle with south korea's first female leader. She too faces equal pressure to be tough. Mr. Kim appears to be more of an extrovert than his dad, however, under his father's watch, four south koreans were killed and 19 injured after a shelling incident launched by the north in 2010, or will he be more like the grandfather he resembles, known for his sabre rattling and ruling with an iron fist? Is there anything you can say with certainty that we know about kim jong-un? The north korean regime doesn't have me on speed dial but what we do know is he is a leader, a young leader recently engaged in provocative talk and provocative actions. Reporter: While u.S. Officials don't expect another immediate show of military force, they are keeping all options on the table, david.

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